Oral sensations associated with food intake influence diet selection in both the short term (i.e., accepting or rejecting ingestible material) and the long term (i.e., nutritional status influences acceptance criteria). They also play an important role in initiating "cephalic phase" responses of the gastrointestinal system, thus regulating the digestive, and therefore the metabolic and nutritive, consequences of food intake.Thus, orosensory function supports processes which have a fundamental impact on health. The arrival of sensory information from the oral cavity at the rostral division of the nucleus of the solitary tract (rNST) is an initial step in these processes. The subsequent steps in processing the sensory input within the rNST are poorly understood. The long-term goal of my research is to understand how chemosensory and somatosensory information is processed within the neural circuits of the rNST and distributed into brain pathways for taste perception, feeding regulation, and gastrointestinal modulation. The inability to associate unique patterns of responsiveness to sensory stimulation, with distinct and identifiable populations of neurons in the rNST has limited progress on this problem in the gustatory system. The aims of this proposal are to identify distinct subpopulations of rNST neurons on the basis of: 1) their cellular responses to activation by electrical stimulation of orosensory nerves and 2) their expression of receptor subtypes for the neurotransmitter released by orosensory nerves (i.e., glutamate). Immunohistochemistry (IHC) for Fos, the protein product of the immediate early gene c-Fos, will be used to identify populations of activated rNST neurons following electrical stimulation of individual gustatory nerves from the tongue. Functional and structural characteristics shared by this group of neurons, or which serve to segregate them into distinct classes, will be identified by combining retrograde tracing, IHC for neurotransmitter expression, and/or morphometrics, with IHC for Fos. IHC will also be used to determine which glutamate receptor subunits are expressed in populations of rNST neurons and if those populations expressing nerve stimulation-evoked Fos demonstrate uniquely identifying glutamate receptor subunit profiles. These data will provide a new means of identifying differences in responsiveness to afferent input between populations of rNST neurons that will be compatible with neuroanatomical and pharmacological methods for analyzing synaptic circuits. The results of these studies will aid analysis of gustatory processing by providing a means of identifying functionally distinct populations of rNST neurons which can be visualized anatomically in analyses of synaptic circuits, and identified functionally in pharmacological studies.